Leaked classified documents show the U.S. National Security Agency and its British counterpart are among the "worst offenders" of mass surveillance without oversight, according to an open letter purportedly written by Edward Snowden and published Sunday by the German magazine Der Spiegel.

The publication of the letter, titled "A Manifesto for the Truth," comes as leaks by the former NSA contract analyst have roiled U.S.-European relations amid allegations that the NSA and the UK's Government Communications Headquarters monitored the communication data of some world leaders.

"The world has learned a lot in a short amount of time about irresponsibly operated security agencies and, at times, criminal surveillance programs. Sometimes the agencies try to avoid controls," Snowden wrote, according to the news magazine.

"While the NSA and GCHQ (the British national security agency) appear to be the worst offenders - at least according to the documents that are currently public - we cannot forget that mass surveillance is a global problem and needs a global solution."

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CNN's Security Clearance examines national and global security, terrorism and intelligence, as well as the economic, military, political and diplomatic effects of it around the globe, with contributions from CNN's national security team in Washington and CNN journalists around the world.